1. Field of the Invention
The present invention relates to a light-emitting diode (LED), and more specifically, to a surface mounted light-emitting diode with high luminous intensity and excellent structural intensity.
2. Description of the Prior Art
Since a light-emitting diode (LED) has the advantages of a long lifetime, a small size, high resistance to earthquakes, and a low consumption of electric power, the LED is widely applied in a pilot lamp or a light source of various household appliances and instruments. Additionally, the LED is developed toward full colors and high brightness, so that the LED is further applied in large-sized display signboards and traffic lights, and it may be substituted for tungsten lamps and mercury lamps in the future.
Please refer to FIG. 1 to FIG. 3. FIG. 1 is a top view of an LED according the prior art. FIG. 2 is a sectional view along line 2—2 of the LED shown in FIG. 1. FIG. 3 is the sectional view along line 3—3 of the LED shown in FIG. 1. As shown in FIG. 1, an LED 10 comprises a printed circuit board 30, and a surface mounted package 11 positioned on the printed circuit board 30. As shown in FIG. 2, the surface mounted package 11 includes a substrate 12 having an upper surface 12a and a lower surface 12b, two composite metal layers 14 and 16 that are insulated from each other and positioned on the substrate 12, an LED chip 18 electrically connected to the composite metal layers 14 and 16, and a sealing member 28 located on the substrate 12 and covering the LED chip 18. As shown in FIG. 1, the LED chip 18 has a p-electrode 20 electrically connected to the composite metal layer 14 through a gold wire 24, and an n-electrode 22 electrically connected to the composite metal layer 16 through a gold wire 26. The p-electrode 20 and the n-electrode 22 are used to receive a driving voltage, so that electrons in the LED chip 18 can be driven by the driving voltage to combine with holes at PN junction for emitting light beams. As shown in FIG. 2, the composite metal layer 14 includes a copper layer 14a positioned on the substrate 12 and extending from the upper surface 12a to the lower surface 12b, a nickel layer 14b positioned on the copper layer 14a, and a gold layer 14c located on the nickel layer 14b. Likewise, the composite metal layer 16 also includes a copper layer 16a, a nickel layer 16b, and a gold layer 16c. Additionally, the substrate 12 comprises a plastic substrate or a ceramic substrate. The sealing member 28 comprises transparent epoxy, and it is used to protect the LED chip 18, and the gold wires 24 and 26.
As shown in FIG. 3, the LED 10 further comprises a plurality of solder paste layers 32 for fixing and electrically connecting the surface mounted package 11 to the printed circuit board 30, and the solder paste layers 32 are not shown in FIG. 1 but shown in FIG. 3 for avoiding confusion. The printed circuit board 30 is used to input the driving voltage to the LED chip 18 for driving the LED chip 18 to emit light beams. The solder paste layers 32 are used to electrically connect the printed circuit board 30 to the gold layer 14c and the gold layer 16c that are positioned on the lower surface 12b. Additionally, the driving voltage supplied by the printed circuit board 30 is input into the p-electrode 20 and the n-electrode 22 through the composite metal layers 14 and 16, and then, the electrons and the holes in the LED chip 18 are driven to be combined at PN junction by the driving voltage for emitting light beams.
As shown in FIG. 3, due to intrinsic characteristics of materials, a binding force between the solder paste layers 32 and the gold layers 14c and 16c is much larger than that between the sealing member 28 and the gold layers 14c and 16c. Accordingly, the sealing member 28 cannot be bonded to the gold layers 14c and 16c quite well, and cracks may be formed between the sealing member 28 and the gold layers 14c and 16c. As the cracks are formed, the solder paste layers 32 would penetrate into space between the sealing member 28 and the substrate 12, as indicated by arrows 34 and 36. Unfortunately, the penetrating solder paste layers 32 may separate the sealing member 28 from the substrate 12, and furthermore, the penetrating solder paste layers 32 may make the gold wires 24 and 26 be broken so the LED chip 18 cannot be driven to emit light beams. Additionally, the gold layers 14c and 16c reflect all kinds of light beams except visible light and light beams whose wavelength is shorter than 400 nm, so that brightness may be degraded when the LED chip 18 is driven to emit light beams of short wave-lengths. Furthermore, because of intrinsic characteristics of materials, a combination between the gold layer 14c and the copper layer 14a is so poor that the nickel layer 14b must be formed between the copper layer 14a and the gold layer 14c to strengthen the structural strength of the composite metal layer 14. Likewise, the nickel layer 16b also must be formed between the copper layer 16a and the gold layer 16c to strengthen the structural strength of the composite metal layer 16. Since it is necessary to form the nickel layers 14b and 16b, manufacturing processes become more complicated and production costs are thereby increased.